If I read things correctly, both buttons and the white box are all in the subform. If so, then script in the hide button referencing the white button by name (relative) would be referencing that instance. If you are using a fully qualified reference without a .all.item(x) in there, then you would be referencing the first one. You could test that by clicking the hide on an added instance and the box appearing in the first instance.
Another possible issue might be that the white box is appearing, but is behind the button on the later instances. You can solve that by hiding the button rather than covering it up. You will probably run into tab issues next anyway where you can tab to the covered up button.